How to Export proportional UV Layout in blender?
Not sure I am understanding your question exactly as default Blender UV export size is 1024 x 1024 or about 27cm on the x and y.

When exporting UVs from Blender, after going to UVs > Export UV Layout there is a little panel in the bottom left where you can modify the output size.



The exporter only takes values in pixels, if you want definite values you can just simply use a converter like this one. translatorscafe.com, just put your desired centimeter value in and it will give you the equivalent in pixels.

As per brecht's comment below, conversion among such units is not as simple as it seems, one key factor involved is the dots per inch (DPI) of your current display. To take that into account, you can use this converter.